Field of the Invention
The present invention pertains to clock recovery in optical storage systems. In such systems, a laser is typically used to read and write data stored in a rotating medium. During these processes, light reflecting from the medium is used for other processes, such as tracking the optical head with respect to the medium, adjusting the rotational velocity of the medium, and recovering clock information.
Recovered clock information is used typically in a phase locked loop (PLL) which locks to a clock reference and generates a write data clock. One issue that arises with such a PLL is the inability to distinguish at high speed the differences between amplitude and phase differences. One approach to handling amplitude differences uses techniques such as automatic gain control (AGC) circuitry to normalize signal amplitude into the PLL. However, for applications such as DVD-RW and DVD-RAM, these AGC structures are too slow. This is especially true given the extreme changes in laser power and therefore in reflected light during the rapid and dramatic changes in laser power from the level needed to read marks up to the levels needed to write marks and back down again.
Another approach is a normalizer circuit which divides the various outputs of the optical detector by the sum of all the detector outputs. However, currently available normalizer designs are too slow to track the rapid changes between the read, write, and erase levels; the use of a normalized signal input to the PLL would introduce signal shifts resulting in phase errors.
To compensate for large signal level changes then, traditional AGC circuitry and normalizer designs are too slow or imprecise. What is needed is a way to stabilize the signal level to the PLL, adjusting for amplitude changes due to the rapid changes in laser power levels from read to write to erase.